The present invention relates to a pulley unit of belt-type continuously variable transmission for vehicles, and specifically to a pulley unit provided with a hydraulic servo device of the construction in which a hydraulic chamber is formed between a movable cylinder secured to a movable sheave and a stationary plunger attached to a pulley shaft.
There has been heretofore generally known a belt-type continuously variable transmission used for vehicles having a construction in which a drive belt composed of a number of metal elements is wound over a primary pulley on an input side and a secondary pulley on an output side and in which each pulley has an annular groove that is variable in width.
Each of the above-described pulleys is provided with a stationary sheave integral with a pulley shaft and a movable sheave fitted on the pulley shaft so as to be shiftable toward and away from the stationary sheave. The width of the groove formed between both the sheaves is varied by shifting movement of the movable sheave. Further, a hydraulic servo device is provided to move the movable sheave toward and away from the stationary sheave.
The hydraulic servo device comprises a movable cylinder secured to the movable sheave and a stationary plunger slidably fitted in the movable cylinder and engaged with the pulley shaft, and a hydraulic chamber is formed between the movable cylinder and the stationary plunger. When the movable sheave is moved toward the stationary sheave due to oil pressure in the hydraulic chamber, the width of the pulley groove is narrowed so as to increase the winding diameter of the drive belt. It is to be noted that on the secondary pulley side, a coil spring is interposed in a compressed state between the movable sheave and the stationary plunger so that even when oil pressure in the hydraulic chamber is released, the movable sheave is pressed toward the stationary sheave by the coil spring to prevent slipping of the drive belt.
The movable cylinder is produced by deep drawing to have a stepped cylindrical configuration for covering the whole stationary plunger, and an enlarged-diameter portion thereof in which the stationary plunger is slidably fitted is secured in a liquid-tight state to an outer periphery of a flange portion projecting annularly on the back surface of the movable sheave.
The pulley unit of the type as described above is disclosed, for example, in U.S. Pat. No. 5,221,235 to H. Ogawa. In this known pulley unit, the large diameter portion of the movable cylinder is caulked to be secured to the outer periphery of the flange portion of the movable sheave. This configuration, however, has a problem in that caulking must be carried out on the enlarged-diameter portion using a special caulking machine, thus increasing the manufacturing cost.
There has been further known a type in which a movable sheave and an enlarged diameter portion of a movable cylinder fitted in an outer periphery of a flange portion thereof are fixedly joined by means such as a beam welding. In this type, however, strains caused by heat treatment is produced in the enlarged diameter portion of the movable cylinder having a thin wall thickness. It is therefore necessary to apply a grinding operation to the inner periphery of the enlarged-diameter portion in order to remove the strains, thus also increasing the manufacturing cost.
Both the known types described above have the construction in which the enlarged-diameter portion of the movable cylinder in which the stationary plunger is slidably fitted is secured to the flange portion of the movable sheave. Therefore, when the diameter of the movable cylinder is changed depending on the type of the belt-type continuously variable transmission, it is necessary to also change the outside diameter of the flange portion of the movable sheave, thus giving rise to a problem in that the movable sheave cannot be used commonly so that the kinds of the movable sheave increase.
Further, in the two known types described above, the movable cylinder having a cylindrical shape that covers the whole stationary plunger is secured to the movable sheave, thus giving rise to another problem in that the internal maintenance of the hydraulic servo device cannot be carried out.